Fluxes of carbon dioxide at Thetford Forest

Abstract. The Thetford Project (1968–1976) was a keystone project for the newly established Institute of Hydrology. Its primary objective was to elucidate the processes underlying evaporation of transpired water and intercepted rainfall from plantation forest, so as to explain hydrological observations that more water was apparently returned to the atmosphere from plantations than from grassland and heathland. The primary approach was to determine the fluxes of water vapour from a stand of Scots pine, situated within a larger area of plantations of Scots and Corsican pine, in Thetford Forest, East Anglia, UK, using the Bowen ratio approach. In 1976, advantage was taken of the methodology developed to add measurement of profiles of carbon dioxide concentration so as to enable the fluxes of CO₂ also to be calculated. A team from Aberdeen and Edinburgh Universities collected 914 hours of 8-point CO₂ concentration profiles, largely between dawn and dusk, on days from March to October, and the data from an "elite" data set of 710 hours have been analysed. In conditions of moderate temperature (<25°C) and specific humidity deficit (<15 g kg⁻¹ with high solar irradiance (>500 W m⁻²), CO₂ uptake reached relatively high rates for pine of up to 20 µmol m⁻² s⁻¹ in the middle of the day. This rate of CO₂ uptake is higher than has been recently found for four Scots pine forests in continental Europe during July 1997. However, the year of 1976 was exceptionally hot and dry, with air temperatures reaching 30°C and the water deficit in the top 3 m of soil at the site of 152 mm by August. Air temperatures of over 25°C led to large specific humidity deficits, approaching 20 g kg⁻¹, and associated severe reductions in CO₂ uptake, as well as in evaporation. However, when specific humidity deficits dropped below c. 15 g kg⁻¹ on succeeding days, generally as a result of lower air temperatures rather than lower solar irradiance, there was rapid recovery in both uptake and evaporation, thus indicating that the large soil water deficit was not the main cause of the reductions in the CO₂ and water fluxes. Based on earlier analysis of evaporation data on completely dry days, the concurrent reductions in CO₂ flux and evaporation are largely attributable to decrease in canopy stomatal conductance. The air temperature, specific humidity deficit, and soil water deficit in 1976 were exceptional and similar conditions have most likely not been experienced again until 2003. We conclude that the information gained at Thetford in 1976 on the response of the pine forest ecosystem to such weather, may provide a good guide to the response of English pine forests to the projected climate change over the next 25 years.